The present invention relates to perfumery and to the flavor industry. It relates more particularly to optically active isomers of alpha-damascone, namely the enantiomer of formula ##STR9## or (-)-(1S)-1-(2',2',6'-trimethyl-2'-cyclohexen-1'-yl)-2-buten-1-one and its -3-buten-1-one derivative.
Ever since their discovery [see Swiss Patents Nos. 509,399 and 524,320; Helv. Chim. Acta, 53, 541 (1970)], the interest encountered by damascones and their derivatives has been steadily growing among perfumers and flavorists. Their organoleptic qualities make them the ingredients of choice in a great variety of compositions destined to a wide range of applications. Their use streches from fine fragrances to the perfuming of household materials or toiletries such as soaps and detergents.
In particular, alpha-damascone has found a large utilization in fruity and floral type composition and its fragrance character of green apple type has enabled the creation of highly appreciated original olfactive notes.
Numerous publications have appeared reporting processes for its preparation. In most of the cases examined, however, these processes revert to the preparation of the racemic compound. alpha-Damascone in effect has been used so far in its racemic form.
G. Ohloff and G. Uhde [Helv. Chim. Acta, 53, 531 (1970)] have however described a process for the preparation of (R)-(+)-alpha-damascone, or (+)-(1'R,E)- and (+)-(1'R,Z)-1-(2',2',6'-trimethyl-2'-cyclohexen-1'-yl)-2-buten-1-one of formula ##STR10##
The process is characterized by an oxidation with manganese dioxide of the corresponding secondary alcohol which was obtained from (+)-alpha-ionone according to the following reaction scheme: ##STR11##
The compound thus obtained showed an [alpha].sup.20.sub.D =+324.degree. for the E form and [alpha].sup.20.sub.D =+340.degree. for isomer Z.
M. Shibasaki et al. [Chem. Pharm, Bull., 23, 279 (1975)] have also described a process for the synthesis of the same isomer of alpha-damascone, which process comprises the diastereomeric cyclization of citral by means of a chiral auxiliary reagent via the formation of its enamines, as indicated hereinbelow: ##STR12##
By making reference to the values given by Ohloff and Uhde, the compound thus obtained by the cited authors possessed an optical purity of 27.5% and showed and [alpha].sup.20.sub.D =+89.2.degree..
We could establish that isomer (R)-(+) thus prepared, while possessing an olfactive note characterized by a pleasant fruity and floral note, showed also a nuance which was reminiscent of "cork". This rendered its utilization somehow problematic.
This observation led us to examine the nature of the odor properties of the other enantiomer, (S)-(-)-alpha-damascone. Unfortunately, in the present state of our knowledge, we were not in the position to synthesize such an enantiomer lacking an appropriate synthetic method.
The present invention obviates this problem.